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Deep crustal assimilation during the 2021 Fagradalsfjall Fires, Iceland

Nature volume 632, pages 564–569 (2024)Cite this article

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Abstract

Active basaltic eruptions enable time-series analysis of geochemical and geophysical properties, providing constraints on mantle composition and eruption processes1,2,3,4. The continuing Fagradalsfjall and Sundhnúkur fires on Iceland’s Reykjanes Peninsula, beginning in 2021, enable such an approach5,6. Earliest lavas of this volcanic episode have been interpreted to exclusively reflect a change from shallow to deeper mantle source processes7. Here we show using osmium (Os) isotopes that the 2021 Fagradalsfjall lavas are both fractionally crystallized and strongly crustally contaminated, probably by mid-ocean-ridge gabbros and older basalts underlying the Reykjanes Peninsula. Earliest eruptive products (187Os/188Os ≤ 0.188, platinum (Pt)/iridium (Ir) ≤ 76) are highly anomalous for Icelandic lavas or global oceanic basalts and Os isotope ratios remain elevated throughout the 2021 eruption, indicating a continued but diluted presence of contaminants. The 2022 lavas show no evidence for contamination (187Os/188Os = 0.131, Pt/Ir = 30), being typical of Icelandic basalts (0.132 ± 0.007). Initiation of the Fagradalsfjall Fires in 2021 involved pre-eruptive stalling, fractional crystallization and crustal assimilation of earliest lavas. An established magmatic conduit system in 2022 enabled efficient magma transit to the surface without crustal assimilation.

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Fig. 1: Location map of the Fagradalsfjall Fires that began in March 2021, showing the 2021, 2022 and summer 2023 episodes.
Fig. 2: Geochemical variations in lavas from the 2021 and 2022 Iceland eruptions as a function of the day of their emplacement, beginning on 19 March 2021.
Fig. 3: Illustration of processes acting on Fagradalsfjall magmas from MgO versus Ni and 187Os/188Os.
Fig. 4: Plots of La/Yb versus Pt/Ir and 187Os/188Os and Os isotopes versus Pt/Ir and δ18O for the 2021 and 2022 Fagradalsfjall Fires.

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Data availability

Source data are provided with this paper in the Supplementary Information and tables. The data are also available at EarthChem (https://doi.org/10.60520/IEDA/113167).

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Acknowledgements

Financial support came from NSF EAR 1918322 (J.M.D.D.) and from the Swedish Research Council and the European Research Council synergy grant (ERC-2023-SyG101118491) (V.R.T.). We are grateful to S. A. Halldorsson for comments that focused arguments in this work.

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Authors and Affiliations

  1. Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA, USA

    James M. D. Day, Savannah Kelly & Geoffrey W. Cook

  2. Department of Earth Sciences, Natural Resources and Sustainable Development (NRHU), Uppsala University, Uppsala, Sweden

    Valentin R. Troll

  3. Centre of Natural Hazards and Disaster Science (CNDS), Uppsala University, Uppsala, Sweden

    Valentin R. Troll

  4. Faculty of Earth Sciences, University of Iceland, Reykjavík, Iceland

    William M. Moreland & Thor Thordarson

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Contributions

W.M.M., V.R.T. and T.T. collected samples. J.M.D.D., S.K. and V.R.T. prepared and analysed all rock samples. J.M.D.D. wrote the first version of the manuscript, with critical input from all authors. All authors contributed to data interpretations, critical discussions and commented on the manuscript.

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Correspondence to James M. D. Day.

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Extended data figures and tables

Extended Data Fig. 1 Plots of Os abundance versus Os isotope composition from Icelandic basalts.

Data are from this study and refs. 19,40,50,51,52,53,54,55,56,57. a, The conservative 50 pg g−1 (or pg/g) cut-off for OIB lavas22. b, Data are all Icelandic historic or prehistoric samples of lavas with >50 pg g−1 Os, defining a range of 187Os/188Os ratios from 0.1263 to 0.1419 (average = 0.1320 ± 0.0072, 2 s.d). The most radiogenic high-Os samples outside the Fagradalsfjall Fires are from the Miocene volcanic rocks of northwest Iceland (ICE-14-16/18 from ref. 50) and so may be subject to notable age corrections. Note the anomalous compositions of the earliest 2021 Fagradalsfjall Fire lavas and the general correspondence of low Os contents with radiogenic 187Os/188Os in the 2021 eruption but not in the 2022 eruption.

Extended Data Fig. 2 Primitive mantle-normalized HSE diagrams.

a, Field of lavas for the 2021 and 2022 Fagradalsfjall Fires versus HSE patterns for prehistoric Icelandic lavas with 9.5–28.0 wt% MgO from ref. 40. Shown is a fractional crystallization model of a primary primitive mantle melt (purple lines) showing compositional variations expected during crystallization. b, Individual 2021 and 2022 Fagradalsfjall Fires HSE patterns versus a model of Ol + Cr + S crystal (red lines) shows the results for 0 to >12% olivine crystallization with co-crystallization of Cr-spinel and sulfide, in the proportions 0.98 olivine, 0.019 Cr-spinel and 0.001 sulfide, using partition coefficients and starting compositions given in Supplementary Data Table 4.

Extended Data Fig. 3 Plot of δ18Omelt versus La/Sm.

Oxygen isotope ratios are plotted as bulk samples (2021 Fagradalsfjall Fires), glass δ18O values or, when olivine was analysed, as melt equivalent based on Δmelt-ol of +0.4‰. Curves a and b represent mixing between a depleted melt with δ18O = +5.2‰ and La/Sm = 0.67 with Krafla basalt with La = 6.3 µg g−1 and La/Sm = 2.9, with δ18O = 0‰ (a) and rhyolitic magma with δ18O = 0‰, with La = 30 μg g−1 and La/Sm = 3.3 (b). Curves c and d are for mixing with the same shallow-level crustal contaminants as given in a and b, respectively, but with a starting melt composition with La/Sm = 2.5 and δ18O = +5.2‰. High-3He/4He ‘enriched’ picrites from northwest Iceland (SEL 97) and central Iceland (NAL 625, PJOR) are shown from ref. 18. Models and published Iceland data (Icelandic lavas) are from refs. 18,19,59,60,61. The O isotope data from ref. 19 have been replaced by that from ref. 34. Note that even the most depleted Fagradalsfjall Fires lavas are not as depleted as some central Icelandic lavas with La/Sm < 1. Later Fagradalsfjall Fires lavas stored at upper crustal depths probably have low-δ18O signatures based on models c and d.

Extended Data Fig. 4 Examples of assimilation of Atlantic gabbro (depleted source of Sr, Nd and Pb) by the early 2021 Fagradalsfjall Fires lavas.

Osmium isotope data indicate that high quantities of assimilated oceanic crust with high Re/Os is permissible, consistent with up to 20% assimilation of such material from Sr–Nd–Pb isotopes. Data in red diamonds and grey squares are from refs. 7,62, respectively, and model parameters and sources are given in Supplementary Data Table 4.

Extended Data Fig. 5 Primitive mantle-normalized HSE diagram for the BHVO-2 standard reference material.

Data from this study and other reported values from the SIGL48,49 are shown versus published data44,45,46,47. Primitive mantle normalization from ref. 26.

Extended Data Fig. 6 A potential model for the magma plumbing system beneath Fagradalsfjall Fires.

Initially, parental magmas originated from polybaric melting to shallow levels, followed by fractional crystallization. Magmas migrated above the Moho in late 2020, at which they fractionated olivine ± clinopyroxene + Cr-spinel + sulfide before eventual mobilization in March 2021. Here they were contaminated by high-Re/Os and high-Pt/Ir melts. As the eruption progressed, progressively deeper portions of the underplating magma feeding zone were involved in the eruption, as suggested by ref. 7. Shown are barometry estimates for crystallization pressures from refs. 7,25 and earthquake focal depth data are summarized from ref. 6.

Supplementary information

Supplementary Data Table 1

New Re–Os isotope and HSE abundance data for the 2021 and 2022 lavas of the Fagradalsfjall Fires.

Supplementary Data Table 2

New trace-element abundance data for the same samples.

Supplementary Data Table 3

New and previously published BHVO-2 standard reference material data.

Supplementary Data Table 4

Model parameters for figures, including refs. 63,64,65,66.

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Day, J.M.D., Kelly, S., Troll, V.R. et al. Deep crustal assimilation during the 2021 Fagradalsfjall Fires, Iceland. Nature 632, 564–569 (2024). https://doi.org/10.1038/s41586-024-07750-0

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